In the design of short pulse laser systems ( less than 2 ps), amplification using the technique of chirped pulse amplification (CPA) has become commonplace. This technique allows high energy, short pulses to be made possible by first stretching the pulse in time, amplifying it, and then recompressing it temporally. In these CPA laser systems, it is required to stretch ultrashort-pulses (or pulses having sufficient bandwidth to become ultrashort) prior to amplification. An ultrashort pulse is one having a duration in the range of 5 femtoseconds to 50 picoseconds.
The ultrashort pulse is stretched to a duration typically greater than 100 picoseconds prior to amplification by the system. Following the stretcher phase, the stretched, amplified, high energy pulse is recompressed to optimally its original temporal length. Such systems typically comprise a reflective system using a high precision grating.
Early designs typically have included at least a pair of diffraction gratings, the first for stretching the ultrashort pulse prior to injection into the laser system, and the second for recompression of the high energy pulse. This leads to costly and complex systems using gratings of the same design because multiple grating stretcher-compressors require precise alignment when a change in laser wavelength is required. In addition, matched grating pairs are required for the stretcher and compressor in order to maintain spatial uniformity and to obtain the appropriate pulse duration.
There is a continuing need for improvements in generating high-energy ultrashort pulses. There is also a need to simplify present systems by making them more compact, cost efficient and less burdensome in alignment sensitivity.
Accordingly, the present invention provides a compact, inexpensive, single common all-reflective stretcher and compressor device for chirped-pulse amplification.
The stretcher in the present all-reflective system comprises optical components that allow the entering beam to strike the all-reflective grating at different vertical displacements during the stretcher path. For example, the entering beam strikes the grating and is diffracted to a reflective focusing optic that reflects the beam to a flat reflective optic at the focal plane of the reflective focusing optic. The beam is then reflected back to the focusing element that will reflect the beam to a vertically displaced position on the reflective grating. The beam is then diffracted to a retro-reflective device such as a roof mirror that again returns the beam to a vertically displaced position of the grating. The beam undergoes additional passes to stretch the beam and exits the stretcher and is amplified before entering the compressor path of the present invention.
The compressor in the present all-reflective system utilizes the same reflective grating element that is used for the stretcher. The amplified beam enters and strikes the grating and is diffracted to a retro-reflective set of optical components that allows translation along the beam path direction to enable compression adjustment of the amplified stretched pulse. The beam is then reflected back to a displaced vertical position on the reflective grating and is interleaved about positions that were used for initial compression of the pulse but is overlapped at those sites on the grating used for pulse stretching. This beam is diffracted to a retro-reflective device such as a roof mirror that again reflects the beam to the reflective grating at a vertically displaced position. The optical path then reverses itself and exits the system as a compressed amplified pulse.
Further aspects of the invention include methods for stretching an ultrashort pulse and compressing an amplified stretched pulse to its former pulse-width by utilizing components that allow overlapping on a common area on a single common reflective diffraction grating.
The present invention provides a cost effective compact system which combines the functionality of a stretcher and compressor employing a single high precision reflective optical grating. The system is an easily aligned, all-reflective, chirped pulse amplification device that can be utilized in commercial systems that need high power for material processing applications.
These features, variations, and other advantages of the present invention will become apparent from the following detailed description, claims and drawings.